A conventional in-process monitoring technique typically employs a two phase inspection procedure. During the first phase the surface of the wafer is inspected at high-speed and relatively low-resolution by e.g. an optical tool or an ebeam tool. The purpose of the first phase is to produce a defect map. For instance, the map may include an array database including X and Y coordinates for each defect (as well as other optional data such as defect type), but may also be otherwise arranged. The number of defects found in the first phase may be very high.
During the second phase only some of the defects found in the first phase may be reviewed, or in other words studied in detail (e.g. by SEM (Secondary Electron Microscope)), and therefore only a small sample of these defects may be selected. Both phases may be implemented by separate tools or by the same tool. A tool for (first phase) inspection and/or for (second phase) review may have a single detector or multiple detectors. Multiple detector two phase inspection devices are described, by way of example, in U.S. Pat. Nos. 5,699,447, 5,982,921, and 6,178,257 whose contents are hereby incorporated herein by reference.
Commonly, the detection of defects is facilitated by comparing measured results, e.g. an image, generated during inspection, with a reference, e.g. on a pixel-to-pixel basis. According to known methods, the reference may be measured results (e.g. images) of a previously measured die (“D2D”—Die to Die techniques), an area in the same die (“C2C”—Cell to Cell techniques), or a statistically generated model based on previously measured data or design data.
In many situations, only a small fraction of the defects detected in the first phase may be reviewed in the second phase (e.g. out of tens of thousands of defects found on a wafer in an optic inspection, less than a hundred may be transferred to a review by a scanning electron microscope (SEM)).
The group of defects which is selected for review is usually selected in order to represent a larger group of defects, based on various characteristics of the defects. For example, the selection may include selecting defects from different parts of the wafer, of different defect type, and so on.
It is noted that a single physical occurrence (e.g. a scratch on the surface of the wafer, uneven polishing of the wafer surface, etc.) may cause thousands of detectable defects. Such a physical occurrence may result in a disproportionately high number of defects (e.g. thousands of defects may result from a single scratch, out of about ten thousand defects in an example wafer). An effective sample of defects selected for review would include just one or a few of these thousands of defects, in order to free more resources for the detailed study of defects resulting from other causes.